1. Field of the Invention
The present invention relates to a spread illuminating apparatus that is used as an illumination unit of a liquid crystal display device or the like, and a method for manufacturing a light guide plate of the spread illuminating apparatus.
2. Description of the Related Art
Liquid crystal display devices are now regularly used as display devices of electronic apparatuses such as personal computers and mobile telephones. Liquid crystal is not a self-luminous display element, and thus in, for example, a transmissive liquid crystal display device, an illumination unit for irradiating light onto a liquid crystal panel is required. Even in a semi-transmissive liquid crystal display device that utilizes external light, an auxiliary illumination unit is provided in order to enable use in dark places. As such the illumination unit for the liquid crystal display device, a spread illuminating apparatus including a light guide plate and a light source disposed to the side of the light guide plate as the main constituent elements is widely used in combination with the liquid crystal display device because such the spread illuminating apparatus has an advantage of being easy to make thin. Further, recent enhancement in the performance of white light-emitting diodes (LEDs) has enabled further reductions in the size, thickness, and power consumption of spread illuminating apparatuses. Thus, spread illuminating apparatuses utilizing white LEDs as a light source are now regularly used.
An example of such a spread illuminating apparatus will now be explained with reference to the constitution (FIG. 8) of a spread illuminating apparatus 10 according to an embodiment of the present invention to be explained later.
The spread illuminating apparatus 10 has a basic constitution in which a light guide plate 12 and LEDs 14 disposed opposing an incident light surface 12c of the light guide plate 12 are accommodated in a housing frame 16. The LEDs 14 are point light sources, and thus when light L emitted from the LEDs 14 enters into the light guide plate 12 from the incident light surface 12c, there are cases in which brightness unevenness referred to as brightness spots or hot spots may occur. Such brightness unevenness may impair the uniformity of the brightness at a light emitting surface 12a of the light guide plate 12. Thus, the following measures have conventionally been proposed for the purpose of overcoming the problem of brightness unevenness.
For example, an example has been proposed in which crimps are formed across the entire surface of the light emitting surface 12a of the light guide plate 12, and the crimps are formed such that the crimp density is low at portions of the light emitting surface 12a in front of the LEDs 14 where the brightness is high and the crimp density is high at portions of the light emitting surface 12a between the LEDs 14 where the brightness is low (for example, refer to JP 2004-145035 A).
Further, in another example, a plurality of grooves with a V-shaped cross section are formed on the emitting surface 12a side of the light guide plate 12 to extend forward from the incident light surface 12c of the light guide plate 12 (toward a light-emitting area side of the light guide plate 12). In this example, the plurality of grooves with the V-shaped cross section are formed such that the depth thereof gradually decreases from the end at the incident light surface 12c side toward the forward direction (for example, refer to JP 2006-171253 A).